(1) Field of the Invention
This invention relates generally to the field of DC-to-DC converters and relates more specifically to buck converters switching with a high flexibility between continuous current mode and discontinuous current mode.
(2) Description of the Prior Art
A buck converter is a step-down DC to DC converter. It is a switched-mode power supply that uses in its basic implementation two switches (usually a transistor and a diode), an inductor and a capacitor.
FIG. 1 prior art illustrates the basic layout of a buck converter. A buck converter comprises a DC voltage source (usually a battery), a switch 2, a diode 3, an inductor 4, often an output capacitor 5, to keep the output voltage constant, and a load 6. The operation of a buck converter alternates between connecting the inductor 4 via switch 2 to source voltage 1 to store energy in the inductor 4 and discharging the inductor 4 into the load 6.
A buck converter operates in continuous current mode (CCM) if the current IL through the inductor 4 never falls to zero during the commutation cycle. The energy stored in inductor 4 increases during On-time of switch 2 (switch 2 closed) and then decreases during the Off-state of switch 2 (switch 2 open). The inductor 4 is used to transfer energy from the input to the output of the converter. The energy stored in the capacitor keeps the output voltage more constant.
In some cases, the amount of energy required by the load 6 is small enough to be transferred in a time lower than the whole commutation period. In this case, the current through the inductor 4 falls to zero during part of the period. The only difference to the continuous mode described above is that the inductor 4 is completely discharged at the end of the commutation cycle, therefore the converter operates in a discontinuous current mode (DCM).
As outlined above, the converter operates in discontinuous current mode when low current is drawn by the load 6, and in continuous mode at higher load current levels. The limit between discontinuous and continuous modes is reached when the inductor current falls to zero exactly at the end of the commutation cycle.
Current practice for buck converters with a continuous current mode (CCM, PWM) is to enter a pulse skipping mode or other mode with discontinuous current (DCM, Pulse Frequency Modulation (PFM)), when the output voltage starts to rise. This works only if negative inductor currents are not allowed and the peak current is limited to a minimum value in continuous current mode. Generally a Schottky diode to ground is used to eliminate negative inductor currents.
It is a challenge for the designers of buck converters to detect the optimal threshold for entering and leaving the discontinuous current mode (DCM) and to implement a fast way for switching back to CCM.
There are patents or patent publications dealing the operation of buck converters:
U.S. patent (U.S. Pat. No. 7,098,632 to Chen et al.) discloses a buck converter in a voltage mode having a pair of switches connected in series by a phase node to be switched by a pair of drive signals generated from a first control signal, a phase resistor is connected between a multifunction pin and the phase node, and a controller generates a second control signal and a third control signal from the second drive signal to sense the voltage on the multifunction pin respectively to generate an over-current signal and a CCM mode switch signal to switch the converter between a CCM mode and a DCM mode.
U.S. patent (U.S. Pat. No. 6,166,528 to Rosetti et al.) proposes a buck converter having a synchronous rectifier topology that performs current sensing at the low-side switch and employs “valley current control” to terminate a discharging phase and commence a charging phase of the converter. The buck converter is able to withstand high operating frequencies and low duty cycles to produce a low output voltage from a given high input voltage.
U.S. patent (U.S. Pat. No. 6,462,963 to Wittenbreder) discloses a tapped inductor buck converter which achieves zero voltage switching and continuous input and output terminal currents. To achieve these results an additional switch, a small inductor, and a capacitor are required. The small inductor serves as a source of energy for driving the critical turn on transition of the main switch and the same small inductor also serves as a filter component for smoothing the input and output terminal currents. Simple adaptive gate drive circuits are revealed that improve the timing for turn on of zero voltage switches and reduce gate drive losses. A synchronous rectifier self drive mechanism is revealed which is universally applicable to zero voltage switching power converters with a single main switch which rely on an auxiliary inductor to drive the critical turn on transition of the single main switch. The wave form generated by the auxiliary inductor is ideally suited to synchronous rectifier self drive. Finally, peak current sensing techniques are revealed which are universally applicable to zero voltage switching power converters with a single main switch and an auxiliary switch which rely on an auxiliary inductor to drive the critical turn on transition of the single main switch. The current sensing techniques sense a winding voltage of the auxiliary inductor during the on time of the auxiliary switch. The winding voltage is directly related to the peak current in the main winding of the auxiliary inductor and the peak current in the single main switch of the power converter. The novel current sensing techniques are low noise, reliable, and lossless.